Microwave Permittivity of Multi-Walled Carbon Nanotubes

Abstract:

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The microwave permittivity of multi-walled carbon nanotubes blended in paraffin wax
has been studied in the frequency range from 2 to 18GHz. The dissipaton factors of the
multi-walled carbon nanotubes are high at the microwave frequencies. The microwave permittivity
of the multi-walled carbon nanotubes and paraffin wax (or other dielectric materials) composites
can be tailored by the content of the carbon nanotubes. And ε′, ε″and tgδ of the composites increase
with the volume filling factor (v) of the carbon nanotubes. The ε′ and ε″ of the multi-walled carbon
nanotubes decrease with frequency in the frequency range from 2 to18 GHz. This property is very
good for broadband radar absorbing materials. The classical effective medium functions can not
effectively model the microwave permittivities of the composites containing multi-walled carbon
nanotubes. The ε′ and ε″ can be effectively modeled using second-order polynomials (ε′,
ε″=Av2+Bv+C). The high ε″ and dissipation factor tgδ (ε″/ε′) of multi-walled carbon nanotubes are
due to the dielectric relaxation. The carbon nanotubes composites would be a good candidate for
microwave absorbing material electromagnetic interface (EMI) shielding material.

Abstract: The electromagnetic shielding properties of short carbon fibers and graphite concrete were investigated, and the influence of the contents of the short carbon fibers and graphite on the electromagnetic shielding effectiveness of the concrete in low frequency was studied. The mechanism causing the influence of the short carbon fibers adulteration on the electromagnetic shielding effectiveness of the graphite concrete at low frequency was investigated. The maximum electromagnetic shielding effectiveness was up to 8.5dB in the low frequency range of 1MHz～1.8GH. The results of the investigation provide the basis for further designing and producing of high-powered electromagnetic shielding concrete at low frequency.

Abstract: Pyramid sharp pyrolysis flame is a new method for carbon nanotubes synthesis. Oxy-acetylene flame outside the frustum of pyramid sharp reactor provides the necessary high temperature circumstance for carbon nanotubes synthesis, while inside the interior mixture of CO, H2, He, and iron pentacarbonyl (Fe(CO)5) is heated. CO is used as the source of carbon, Fe(CO)5 as the source of catalyst precursor. Special structure of the frustum of pyramid sharp reactor makes the oxy-acetylene flame folded gradually above the reactor. And it meets the condition that the interior mixture which has reacted initially under high temperature and will flow out of reactor avoids exposing to air completely and burning abundantly. Immersing a sampling substrate into the incomplete burning flame can gain carbon nanotubes. By adjusting the distance between the oxy-acetylene flame jet and the synthesis area, achieved the purpose that just changing one factor of synthesis or pyrolysis temperature while the other one constant, then respectively studied the effects of them on experimental. The perfect synthesis temperature in experimental is about 595°C, while the pyrolysis temperature is about 1000°C.

Abstract: Surface modification of carbon fibers(CF) by physicochemical methods directs an attractive approach for improvement of metal uptake from solutions. We investigated pretreatment of carbon fibers by HNO3 with different time on absorption of catalysts, which is related to the coverage of carbon nanotubes (CNTs) grown on Carbon fibers. The effects of surface modifications on the properties of carbon fibers were studied by X-Ray photoelectron spectroscopy. The modifications bring about variation in the chemical properties. The modification increased a large number of surface functional groups such as hydroxyl and carbonyl. The HNO3 modification increases the catalysts absorption. The coverage of CNTs on CF increases with pretreatment time, which was studied by SEM.

Abstract: The field emission properties of single-wall carbon nanotubes with purity higher than 70%,which were produced by dc arc discharge evaporation of a carbon electrode including 1% Fe catalyst in H2-Ar mixture gas was studied.The single-wall carbon nanotubes that were produced by this method possess high a ‘clean’ surface since the coexisting Fe catalyst nanoparticles can be completely eliminated by a two-step purification process.The field emission properties was verified by measuring the emission current density(J) versus the applied electric field(E), the corresponding Fowler-Nordheim(F-N) plot for the sample.Through looking at the emission photos, the uniformity of field emission is found to be excellent.

Abstract: Electromagnetic parameters and absorbing properties of fiber absorbents (carbon fiber, SiC fiber and polycrystalline iron fiber) were introduced. The influences of the arrangement, thickness and content of the fibers on radar absorbing property were summarized. New development directions of the fiber absorbents were also indicated.